The present invention relates to an electric field level detecting apparatus for detecting the reception electric field level of a receiver.
In general, in a mobile radio communication system such as a mobile telephone system, the reception electric field level of a reception signal must be always monitored to hand over a mobile station from a base state in communication to an adjacent base station. In order to monitor the reception electric field level, the mobile station comprises an electric field level detecting circuit for detecting the reception level of a reception signal from at least one of the base station in communication and the adjacent base station.
A conventional electric field level detecting circuit, as shown in FIG. 5, comprises a radio frequency amplifier 22 for amplifying a radio frequency reception signal input from an antenna 21, a mixer circuit 23 for converting an output from the radio frequency amplifier 22 into a first intermediate-frequency signal a, an intermediate-frequency amplifier 24 for amplifying the first intermediate-frequency signal a to output a second intermediate-frequency signal b, a limiter amplifier circuit 25 for receiving the second intermediate-frequency signal b as an input and outputting an analog voltage c corresponding to the level of the second intermediate-frequency signal b, an analog/digital converter 26 for converting the analog voltage c into a digital voltage d, and a table 27 for receiving the digital voltage d as an input and causing a reception electric field level indicating the electric field strength of the radio frequency reception signal to correspond to the digital voltage d.
The operation of the conventional electric field level detecting circuit will be described below.
The radio frequency reception signal input from the antenna 21 is amplified by the radio frequency amplifier 22 and converted into the first intermediate-frequency signal a by the mixer circuit 23. This first intermediate-frequency signal a is amplified as the second intermediate-frequency signal b by the intermediate-frequency amplifier 24 and input to the limiter amplifier circuit 25.
The limiter amplifier circuit 25 outputs the analog voltage c corresponding to the level of the second intermediate-frequency signal b, and the analog voltage c is converted into the digital voltage d by the analog/digital converter 26. The reception electric field level is read out from the table 27 which causes the reception electric field level indicating the electric field strength of the radio frequency reception signal input from the antenna 21 to correspond to the digital voltage d , thereby detecting an electric field strength.
In this conventional electric field level detecting circuit, when a radio frequency reception signal having a high electric field strength is input, the intermediate-frequency amplifier 24 is saturated. For this reason, the relationship between the electric field strength of the radio frequency reception signal input to the antenna 21 and the second intermediate-frequency signal b output from the intermediate-frequency amplifier 24 has nonlinearity, and an electric field level cannot be correctly detected.